Two disparate genres smash together in “Colossal,” a Kaiju monster movie mixed with a dramedy about a down-and-out woman returning to her small-town home and reconnecting with a childhood friend.

This shouldn’t work at all and it’s a testament to the conviction with which writer-director Nacho Vigalondo and star Anne Hathaway pursue the unlikely premise that it does, in fits and starts at least, though the clashing modes end up overwhelming things a bit too frequently.

Hathaway makes the wise decision to stretch out of her comfort zone and reconstitute her identity as an actor by playing Gloria here, kicked out of her New York City apartment by her boyfriend (Dan Stevens). Bottoming out, with nowhere to turn, she moves back home and gets a job in the scuzzy bar run by Oscar (Jason Sudeikis).

That sounds like one sort of picture, yet another dour variation on the “Garden State”/ “Return of the Secaucus Seven” nostalgia model. A lot of “Colossal” plays as precisely that, with characters who are content to do little but mope and drink and shoot the breeze in a generic American hellscape.

Of course, the complicating factor comes in the form of a Kaiju attacking Seoul, South Korea, concurrent to Gloria’s downward spiral, and the weight of the increasing realization that there’s a deep connection between the monster and the protagonist.

It’s best to not reveal exactly what that is here, or how it’s developed and portrayed, though it amounts to the essence of this movie, a metaphor for self-empowerment and inner freedom that packs a certain degree of power, even as it provides a cautionary tale of the limits that come with structuring an entire dramatic enterprise around an abstraction.

Vigalondo presents Hathaway and, to a lesser extent, Sudeikis, with an immense challenge: scenes packed with double and triple meanings; severe personal consequences; and the threat of broader deadly destruction. The movie marks a new beginning for Hathaway, a very good actress who had unfairly started to become an object of scorn. She delves completely into the zaniness while keeping Gloria rooted in tangible emotional currents, and perhaps the most exciting thing about the whole enterprise is what it suggests about her future.

Perhaps inevitably, “Colossal” plays better as a high-concept experiment than a unified motion picture. The bridge between its worlds never solidifies to the point where it’s possible to fully accept and go with what Vigalondo is doing. It’s a study in contrasts, a B-movie on one hand and an earnest drama with a mean streak on the other, and neither mode is fleshed out enough to really matter. But the ambition is inspiring.